1. Field of the Invention
The present invention relates to a halogen-free varnish and a prepreg thereof, in particular to a halogen-free varnish with composite curing agent, condensed phosphate of aromatic compound, and filler; wherein a prepreg may be formed by dipping glass fabrics into the halogen-free varnish.
2. Description of Related Art
Printed circuit board (PCB) is widely used in many applications and fields, for example, electronic devices of electronic products are inserted on PCBs. Now, PCBs are increasingly applied to high power and high thermal environment. Therefore, the study and research on heat-dissipation are developed in order to improve the heat-dissipation efficiency of PCBs.
PCB is made by pressing the prepregs, copper clad laminate (CCL), or copper film. The prepregs are made by dipping glass fabrics into varnish and then curing the fabrics to form thin prepregs.
Regarding environmental protection, halogen-free PCB is more and more developed for electronic application. The curing agent for the epoxy resin used to manufacturing the halogen-free PCB is classified into dicyandiamide and phenol novolac resin systems. The prepreg made of curing agent of dicyandiamide has disadvantage of low heatproof and high moisture absorption. The prepreg made of curing agent of phenol novolac resin system has better properties than the prepreg made of dicyandiamide curing system. However, the curing agent of phenol novolac resin has to be developed to work with the different epoxy resin. Patent NO. TW 293831 disclosed a varnish with Benzoxazine (Bz) resin and condensed phosphate compound in order to manufacture prepregs with anti-flammability property. Further, Patent NO. TW 583258 disclosed a varnish with curing agent of Bz resin and amino triazine novolac (ATN) resin in order to improve the anti-flammability of prepregs. However, Patent NO. TW 583258 disclosed Bz resin of higher parts than ATN resin, and the higher composition of Bz resin results in high glass transition temperature (Tg) and lower moisture absorption, but lower heatproof characteristic (Solder Float Resistance testing). Moreover, the toughness of the prepreg is decreased and the over-addition of Bz resin will decrease the heatproof characteristic
Therefore, in view of this, the inventor proposes the present invention to overcome the above problems based on his expert experience and deliberate research.